Pressurized coal gasification is not only the key part of the Integrated Gasification Combined Cycle (IGCC) but also the better approach to increase the gasification reaction rate and capacity of the unit gasifier. In this work, the gasification characteristics for a sub-bituminous coal with an air-steam mixture at the gage pressure of 0.5-1.4 MPa and the temperature of 880-980 degreesC in a 200 mm inside diameter pressurized fluidized bed gasifier have been investigated. Results show that the gasification process is dominant in the transition zones between chemical reaction (or pore diffusion) and gas film diffusion at the pressure of 1.0 MPa. For the sub-bituminous coal tested, the increase of the capacity of gasifier is almost in linearity with pressure up to 1.2 MPa, and then it slows down because of transferring to gas diffusion of gasification reaction. In addition, the effect of pressure on the gas composition and the size distribution of bed holdup and fly ash are discussed in detail.